The invention relates to an oscillating platform on flexible piles for work at sea, the piles being fixed to the upper part of the lattice tower and being driven into the sea bottom so as to maintain the tower; means being provided for resisting tower twistings movements.
Oscillating platforms in which the articulation connecting the base to the lower end of the tower has been eliminated and replaced by a number of piles driven into the sea bed and extending to the upper part of the tower to which they are secured, are known. Such a platform is disclosed in U.S. Pat. No. 4,417,831. In this construction, the piles extend freely within a number of legs of the tower or in guides fixed to the bracings. The tower is anchored to the sea bottom by guying, so as to ensure that the current and wave effects have not an excessive influence on the lateral movements of the tower.
Permanent floaters may be used for supporting part of the weight of the deck, and more particularly for resisting excessive loads and forces in the piles during strong storms. They are unnecessary under normal environmental conditions.
When the platform is used to put in production pre-drilled wells from a sub-sea template, anchoring piles of the template are provided for receiving some of the legs of the tower. The purpose of this attachment is to avoid torque resulting from a dissymetry of the forces (wind, current, etc.) applied to the tower.
Such guyed platforms, articulated to flexible piles have allowed a reduction in the masses, and therefore costs, owing to the elimination of the articulation (universal or swivel joints) and its supporting base. On the other hand, the guying system required for the stability led to multiply the number of anchoring points and to increase the maintenance requirements. The control of the torque effect through part of the legs of the tower induces dissymmetrical forces in the structure which result in an absornal fatigue in some elements and adverse effects on the life of the structure.